The principle behind the screen is simple: robber bees are attracted
to the scent of the colony they wish to rob, though they aren't
oriented to it. By moving the entrance 6 inches up, but allowing
the scent to issue from the same place, the robbers will keep bonking
away at the screen until they give up. The inside worker bees, however,
will eventually figure it out as they orient themselves when they
emerge, and when the field bees finally stumble in the right place.

If you look at the picture top left, you see what looks alot like
a bad picture frame holding hardware cloth with openings too small
for a bee to pass through. On the inside (facing the hive) top and
bottom edges, however, the frame is missing (or has openings). When
that side is held up against the front of the colony, the bees can
descend though the opening at top and enter the hive through the
one on the bottom.

My screen is not the same as the model, because I do not have real
woodworking tools: instead, a mini-mitre box worked to cut already-milled
1/2" dowels, and some wire snips managed to cut a piece of
1/4" hardware cloth to size. My staple gun tacked it all together.
The black rope in the photo on the right is actually one of my extensive
collection of bungee cords.

I had to squash some bees to install it, and this felt just horrible.
The bees — many of them carrying pollen — became totally
confused and just ran all around after I installed. This was good,
in that robber bees do not bring pollen to hives: meaning that the
bees at the entrance were no longer robbers. Also good: apparently
our pollen situation is good even though nectar is bad. JonM, the
beekeeper who helped me, says that the real field bees will work
out how to enter eventually, while the frenzied robbers never will.